The human T-cell leukemia virus type-1 (HTLV-1) is acomplex retrovirus etiologically linked to an aggressive and generally fatal malignancy called adult Tcell leukemia (ATL). The molecular basis of HTLV-I associated oncogenesis is linked to the expression of the virally-encoded Tax protein. Robust transcription of the HTLV-I genome requires Tax in cooperation with the cellular transcription factor CREB (or other members of the ATF/CREB family). Three 21 base pair repeat enhancer elements, located in the HTLV-I transcriptional control region, are critical to Tax-activated transcription. These elements, which are referred to as viral CREs, are binding sites for the Tax/CREB complex. The formation of this Tax-containing promoter-bound complex is critical in the recruitment of the multifunctional cellular coactivators CBP/p300. CBP and p300 are very large, highly related proteins that posses acetyltransferase activity and coordinate highly regulated gene expression in metazoans. Several conserved domains in CBP and p300 serve as binding sites for a wide variety of cellular and viral transcription factors. The transcription factor coactivator interactions mediated by these domains facilitate gene-specific recruitment of the coactivators. Recent studies in several laboratories have demonstrated the KIX domain of CBP/p300 is utilized, perhaps exclusive, in the recruitment of the coactivators to the HTLV-I promoter. This is accomplished via direct binding of KIX to the viral CRE-bound Tax/CREB complex. Once recruited, several studies have shown that CBP/p300 mediate Tax-transactivation via the acetyltransferase activity of the coactivators. Recent experiments in our laboratory demonstrate that the acetyltransferase activity is not directed at the histone amino terminal tails. Instead, we find that non-histone proteins, including Tax and CREB, are the direct acetylation targets of p300. Significantly, Tax/CREB acetylation by p300 is dramatically stimulated when both proteins are in complex with the viral CRE DNA. In this application, we propose to determine the three dimensional structure of the Tax/CREB/viral CRE complex bound to the KIX domain of CBP. This will be carried out in collaboration with Dr. Karolin Luger, who participated in the structure determination of the nucleosome core particle. The additional aims will focus primarily on the biochemical characterization and functional analysis of CBP/p300 acetylation of the Tax/CREB complex. Acetylation of lysine residues on nonhistone transcription factors is rapidly emerging as a prominent regulatory mechanism in eukaryotic cells.